Cinnamic acid, or 3-phenyl-2-propenoic acid, is an important industrial chemical. Cinnamic acid and its derivatives are used as sun screening agents, flame retardants, cosmetics, fungicides, insecticides, food preservatives, pharmaceuticals and photographic agents. Cinnamic acid has also received considerable attention recently since it is a precursor of phenylalanine, one of the ingredients in the artificial sweetener aspartame.
The most widely used commercial process for the production of cinnamic acid has used benzaldehyde, acetic anhydride and anhydrous sodium or potassium acetate in a Perkin condensation reaction. In another commercial process for the production of cinnamic acid, benzal chloride and anhydrous sodium acetate are heated to 180.degree. to 200.degree. C. Since benzal chloride is cheaper than benzaldehyde, this method is especially favored by manufacturers who obtain by-product benzal chloride from their benzyl chloride plants. This basic reaction, disclosed in German Pat. Nos. 17467 and 18251, required severe reaction conditions, namely 20 atmospheres pressure and temperatures above 200.degree. C. for 10-20 hours. These severe reaction conditions are undesirable due to the attendant dangers associated with high pressure as well as for economic reasons. Furthermore, large quantities of a tarry component are produced under the above reaction conditions.
An improvement in the above described process is reported in Japanese Pat. No. 48-81830 (1973) wherein the inventors in this Japanese application disclose that potassium acetate can be reacted with benzal chloride under normal pressure to produce high yields of cinnamic acid if an amine such as pyridine, quinoline or aniline is used as a catalyst. The process reported in the above Japanese patent is not easily reproducible. Present attempts to reproduce the results reported in this patent have resulted in yields of cinnamic acid significantly lower than those reported by the Japanese inventors. Moreover, under the conditions reported in the above patent, a difficultly stirrable reaction mixture is produced. In the process disclosed in the above Japanese patent, the preferred amount of potassium acetate is 4-5 moles per mole of benzal chloride. The Japanese inventors disclose that 3 moles can be used but regardless of the molar amount of potassium acetate used, the above process still requires the use of large amounts of the expensive potassium salt for the desired yields.
The use of condensing agents in Friedel-Crafts synthesis is also well documented. A complete discussion of this subject can be found in the article entitled "The Friedel-Crafts Synthesis", Chem. Rev. 17, p. 376, 1936.